1. Field of the Invention
This invention relates to devices employing high T.sub.c superconducting materials, and more particularly to simple, practical devices employing these materials, and to methods for making these devices. The devices are planar structures employing selected grain boundaries in the high T.sub.c superconducting materials as weak link or junction barriers. Such structures are reproducibly made with good operating margins.
2. Description of the Prior Art
Recently, the remarkable discovery by J. G. Bednorz and K. A. Mueller, reported in Z. Phys. B.--Condensed Matter 64, 189 (1986) and Europhysics Letters, 3, 379 (1987) completely changed the direction and importance of superconducting technology. Their discovery was that certain metallic oxides can exhibit superconducting transition temperatures considerably in excess of 23.degree. K. These materials are often termed "High T.sub.c Superconductors". Since the initial discoveries of Bednorz and Mueller, a vast amount of research and development has been undertaken around the world to further study these types of superconducting materials in order to extend even farther the temperature range over which the materials are superconducting, as well as to understand the basic mechanisms for superconductivity in this class of materials.
Bednorz and Mueller first showed superconducting behavior in mixed copper-oxides, typically including rare earth and/or rare earth-like elements and alkaline earth elements, for example La, Ba, Sr, . . . , and having a perovskite-like structure. Materials including the so called "1-2-3" phase in the Y-Ba-Cu-O system have been found to exhibit a superconducting transition temperature in excess of 77 K. R. B. Laibowitz and co-workers were the first to achieve and describe a method for making thin films of these materials. These thin film structures and methods for making them are described in co-pending application Ser. No. 027584, filed Mar. 18, 1987 and assigned to the present assignee. The work of Laibowitz et al is also described in Phys. Rev. B, 35, 8821 (1987). In this technique, a vapor transport process is used in which the components of the superconducting film are vaporized and deposited on a substrate in an oxygen atmosphere, after which the deposited film is further annealed.
Another paper describing thin films of these high T.sub.c superconductors, and specifically high critical currents in these materials, is P. Chaudhari et al, Phys. Rev. Lett. 58, 2864, June 1987. Chaudhari et al described epitaxial high T.sub.c superconducting films formed on substrates such as SrTiO.sub.3, in which the critical current at 77 K. was in excess 10.sup.5 A/cm.sup.2.
Other references generally describing the deposition of films or layers of high T.sub.c superconducting materials include J. Cuomo, co-pending application Ser. No. 043,523, filed Apr. 28, 1987, and A. Gupta, co-pending application Ser. No. 121,982, filed Nov. 18, 1987, and also assigned to the present assignee. The first of these co-pending applications describes a plasma spray coating process while the second describes a method for coating a substrate, as by spraying from solution, and then patterning the coated film to eventually produce a patterned layer of high T.sub.c superconducting material.
Epitaxy of high T.sub.c superconducting films has been accomplished on several substrates, including SrTiO.sub.3. In particular, superconducting films capable of carrying high critical currents have been epitaxially deposited as noted in a paper by P. Chaudhari et al, published in Physical Review Letters, 58, 2684, June 1987.
The initial work of Bednorz and Mueller has been extended to include other copper oxide compositions which exhibit high temperature superconductivity. These other compositions typically do not include a rare earth element, but instead include an element such as Bi. A representative material is one in the system Bi-Sr-Ca-Cu-O which exhibits a drop in electrical resistance at about 115 K. and a transition to zero resistance at 80 K. Recently, C. Michel and co-workers reported superconducitivity in the non-rare earth containing BiSrCuO system with transition temperatures as high as 22 K. C. Michel et al, Z. Phys. B-condensed Matter, 68, 412 (1987). A new BiSrCaCuO.sub.x composition was then found by Maeda and Tanaker to exhibit high transition temperatures with a resistivity completion in the 80 K. range and a well defined resistivity decrease at about 115 K. This work was reported by these authors in the Japanese Journal of Physics Volume 27, Pages L209-L210, (February 1988).
The work of Maeda and Tanaker was confirmed by C. W. Chu and co-workers, and by Hazan and co-workers, these researchers noting the structure and phase identification of this bismuth-including copper oxide system. Reference is made to C. W. Chu et al, Phys. Rev. Lett., Volume 60, pages 941-943 (Mar. 7, 1988), and R. M. Hazen et al, Physical Review Letters Vol. 60, Pages 1174-1177 (Mar. 21, 1988).
The copper oxide superconducting materials exhibiting transition temperatures in excess of about 30.degree. K. are generally known as "high T.sub.c superconductors", and will be referred to in that manner throughout the specification. This designation is meant to include both the materials having rare earth or rare earth-like elements in their crystalline structure, as well as the more recently reported materials which do not contain rare earth or rare earth-like elements. Generally, all these materials are copper oxide based superconductors having Cu-O planes that appear to be primarily responsible for carrying the supercurrents, where the copper oxide planes are separate or in groups separated by the other elements in the compositions.
The advent of high temperature superconductivity should lead to numerous applications of junction devices operating at temperatures much high than those that have been achieved with superconducting devices fabricated from conventional superconductors. However, fabrication of workable devices has not been easy. The first such report of an operable device, in this situation a SQUID device described by R. Koch et al, utilized a film of high T.sub.c superconductor in which high energy beams were used to produce two localized constrictions to form weak link connectors between high T.sub.c superconductors. In this manner, a superconducting loop having weak link regions was created and operated successfully as a SQUID. This first high T.sub.c superconducting device and the method for making it are described in a copending patent application to G. J. Clark et al, Ser. No. 7-037912, filed Apr. 13, 1987 and assigned to the present assignee.
Although it has been experimentally established that high T.sub.c superconducting copper oxides, such as YBa.sub.2 Cu.sub.3 O.sub.7-x can be reproducibly prepared as thin films, a well defined, all high T.sub.c single junction exhibiting Josephson tunneling currents has not been successfully fabricated. In such a device, two superconducting layers comprised of high T.sub.c superconductors are separated by a thin (10-50 angstrom) layer which operates as a tunnel barrier. An oxide material can be used for the barrier layer. However, the high T.sub.c copper oxide superconductors, whether fabricated as films or bulk samples, require annealing in an oxygen atmosphere at high temperatures, typically about 900.degree. C. This high temperature processing makes it extremely difficult if not impossible to deposit a counter electrode comprised of high T.sub.c superconducting material over the very thin insulating tunnel barrier. Generally, the high temperature processing severely degrades the junction quality. Such processing is also incompatible with most of the conventional lithographic patterning processes.
Another feature of these high T.sub.c superconducting materials is their extremely short coherence length, which is a measure of the distance over which the superconducting pairing extends. In these high T.sub.c superconductors the coherence lengths are typically 10-30 angstroms, in contrast with coherence lengths of 1000 angstroms in conventional prior art superconducting materials. Such low coherence lengths represent another technical obstacle to making either planar function or weak link type tunnel barriers in, for example, microbridge Josephson junction devices. In weak link devices, a very narrow constriction operates as a weak link barrier between two large superconducting regions to provide Josephson-like characteristics. However, because the coherence length is so small in high T.sub.c superconductors, the geometrical constriction must have a dimension of the order of the coherence length in order to exhibit weak-link characteristics. Such narrow constrictions cannot be reliably produced. When planar junctions are formed, it is also very difficult to reliably deposit tunnel barrier layers having thicknesses of the order of the coherence length (about 10 angstroms) of high T.sub.c superconductors.
Accordingly, it is a primary object of the present invention to provide a practical device employing high T.sub.c superconducting materials where the aforementioned problems are avoided.
It is another object of the present invention to provide a method for reproducibly making practical junction and weak link superconducting devices employing only high T.sub.c superconducting materials.
It is another object of this invention to provide a device and method for making the device employing high T.sub.c superconducting materials in a planar configuration wherein the weak link or junction region can be precisely located with a defined orientation.
In the practice of this invention, junction devices or weak link devices are fabricated using a grain boundary between two high T.sub.c superconducting grains. These grain boundaries are very narrow (about the order of the unit cell in the materials, i.e., about 10 angstroms) and their electrical properties (such as resistance) can be readily varied to provide different device properties. In particular, a planar structure is provided utilizing an epitaxial film of high T.sub.c superconducting material deposited on a substrate having defined and predetermined grain boundaries therein. In this manner, the grain boundaries in the substrate are reproducibly formed in the epitaxial superconductor film. Stated another way, epitaxy maps the polycrystalline structure of the substrate into the high T.sub.c superconductor film.
It is recognized that grain boundaries have been used to provide potential barriers for the flow of electrons thereacross in prior superconducting devices. Such devices have been called boundary layer Josephson junctions and have been described in the following references:
M. Ito et al, Japanese Journal of Appl. Physics, 21, No. 6, pp. L375-376, June 1982.
M. Ito et al, Appl. Phys. Lett., 43, (3) p. 314, August 1983.
T. Inamura et al, Jap. Journal of Appl. Phys., 21, Supplement 21-1, pp. 313-318, 1982.
The devices described in these references use the grain boundaries that randomly occur when a superconductor film is deposited on a substrate. These superconductors are generally designated BPB films because they are comprised of Ba, Pb, and Bi oxide combinations having a perovskite-type structure. These references do not teach a way to controllably make grain boundary junction devices whose characteristics can be well controlled and which can be reproducibly formed with uniform properties. As noted, these references describe devices in which a random formation of randomly oriented grains occurs in materials having low transition temperatures of about 13 K.
In further contrast with these and other references, the devices of the present invention are made in an epitaxial layer of high T.sub.c superconducting material. Generally, epitaxy is thought of with respect to single crystal material rather than polycrystalline materials of the type used for the substrate and the superconducting film in the devices of this invention.
M. Suzuki et al describes the formation of planar Josephson-type devices using crystalline layers of BPB in J. Appl. Phys. 53, (3), p. 1622, March 1982. In this structure, two superconducting layers of BPB are separated buy an insulating tunnel barrier comprised of an insulating oxide having the same crystal structure as BPB. Such device structures have not been possible using high T.sub.c superconducting materials, for the reasons described above with respect to the high temperature processing and very short coherence length in these new superconductors.
Accordingly, it is another object of the present invention to provide practical devices utilizing selected grain boundaries in high T.sub.c superconducting materials.
It is another object of this invention to provide processing techniques for reproducibly making grain boundary superconductive devices employing high T.sub.c superconducting materials, wherein the device properties are uniform and wherein the device structures are planar and easily and reproducibly fabricated.
It is another object of this invention to provide improved devices employing high T.sub.c superconductors, wherein the design of such devices and the techniques for making them effectively utilize features found in nature which may otherwise be considered obstacles.